Hello. I have a toilet that is very mercurial in a house I recently purchased. Some flushes (10%) it works exactly as designed, some the water swirls around the bowl and never really drains fully but definitely doesn't back up either (80%), and some the water drains slowly and nearly fully empties the bowl (10%). I've examined the tank and mechanically everything appears to be working properly. There was *a lot of black sediment* and some rust build up that I've nearly completely removed. I've also used a 6' snake to clear any blockage. It did not help at all. Finally, assuming it was a blockage, perhaps further than my 6' snake could reach, I filled a 5 gallon bucket of water and poured the whole thing down the toilet--it drained perfectly. This says to me there is no blockage issue. So what's the likely problem? I'm stumped.